1. Field of the Invention
The present invention relates to a light source apparatus capable of changing an oscillation wavelength and an image pickup apparatus using the same.
2. Description of the Related Art
A variety of light sources, in particular, laser light sources, which are capable of changing an oscillation wavelength have been utilized in a communication network field and a field of an inspection apparatus.
In the communication network field, a high-speed wavelength switching is demanded, and also, in the field of an inspection apparatus, a high-speed broadband wavelength sweeping is demanded.
Use applications of a wavelength-variable (sweeping) light source in the inspection apparatus include a laser spectrometer, a dispersion measurement device, a film thickness measurement device, a swept source optical coherence tomography (SS-OCT) apparatus, and the like.
The optical coherence tomography (hereinafter, which will also be referred to as OCT) is configured to pick up a tomographic image of a sample by using an optical low-coherence interference, and this is an image pickup technology whose research in a medical field has been activated in recent years because a micron-order spatial resolution can be obtained, a noninvasive property is attained, and the like.
Currently, the OCT is used for an image pickup in eye clinics, an image pickup in dental clinics, or the like as a resolution in a depth direction is set as several microns and also a tomographic image up to a depth of several mm can be obtained.
The SS-OCT is designed to temporally sweep an oscillation wavelength (frequency) of a light source and falls within a category of a Fourier domain (FD) OCT. A spectrum domain (SD) OCT that also falls within the category of the FDOCT needs a spectrometer for dispersing an interference light, but the SS-OCT is also expected to obtain an image in which a loss of a light amount is small and an SN ratio is high as the spectrometer is not used.
In a case where a medical-use image pickup apparatus is composed by using a wavelength sweeping light source, a period of time for obtaining an image can be shortened as a sweeping speed is faster, and the apparatus is suitable to a vital observation (so-called, in situ-in vivo imaging) in which a biomedical tissue is not collected from a living body and the biomedical tissue is directly observed.
Also, as a sweeping band of the wavelength is wider, the spatial resolution of the tomographic image can be increased.
To be more specific, a depth resolution is represented by the following expression (1) while the wavelength sweeping width is set as Δλ, and an oscillation wavelength is set as λ0.
                    Expression        ⁢                                  ⁢        1                                                                                  2            ⁢            ln            ⁢                                                  ⁢            2                    π                ×                              λ            0            2                                Δ            ⁢                                                  ⁢            λ                                              (        1        )            
Therefore, in order to increase the depth resolution, an expansion of the wavelength sweeping width Δλ is needed.
For the wavelength sweeping light source used in the SS-OCT, one based on a dispersion tuning system which is developed in a communication band is disclosed in Yamashita, et al. Opt. Exp. Vol. 14, pp. 9299-9306 (2006) (hereinafter, which will be referred to as “Non-patent Document 1”).
In this dispersion tuning system, by utilizing a state in which a free spectral range (FSR) of the resonator has a wavelength dependence, the oscillation wavelength in an active mode-lock state is controlled. At this time, the wavelength sweeping is performed by changing a frequency of a modulation signal that may cause a mode-lock. In other words, by sweeping a mode-lock frequency, the central wavelength at the time of the mode-lock is swept. For this reason, in order to perform a high-speed wavelength sweeping, it is necessary to change the frequency of the modulation signal at a high speed.
The FSR indicates a frequency interval in a resonator mode with respect to light circulating in the resonator and is represented by the following expression (2) while the light speed in vacuum is set as c, a refractive index of the resonator is set as n, and a resonator length is set as L.
                    Expression        ⁢                                  ⁢        2                                                            FSR        =                  c          nL                                    (        2        )            
Also, according to Non-patent Document 1, in the dispersion tuning system, a wavelength sweeping range is represented by the following expression (3).
                    Expression        ⁢                                  ⁢        3                                                                      Δ          ⁢                                          ⁢          λ                =                  n          cDN                                    (        3        )            
Where n denotes the refractive index of the resonator, D denotes a dispersion parameter of the resonator, and N denotes an order (natural number) of the mode-lock.
Also, aside from this, for a purpose of an optical wavelength division multiplexing, a mode-lock laser technology for constructing an apparatus provided with a plurality of gain materials in a resonator and carrying out pulse oscillations in a plurality of spectrum bands at the same time is proposed in Japanese Patent Laid-Open No. 6-90050 (hereinafter, which will be referred to as “Patent Document 1”).
According to the dispersion tuning system based on the active mode-lock disclosed in the above-described Non-patent Document 1, the wavelength sweeping can be performed, but the sweeping range is limited to a relatively narrow range in principle, and also the sweeping speed is not necessarily sufficient. Also, wavelength sweeping notches at the time of the sweeping are not constant as the FSR has the wavelength dependence, and in order to obtain a quasi-smooth sweeping, a devise on a detection side is demanded.
On the other hand, according to the laser apparatus disclosed in Patent Document 1, oscillations at a plurality of wavelengths can be carried out, but it is simply assumed that the oscillations are carried out at a plurality of wavelengths at the same time point, and an intention of performing the wavelength sweeping does not exist.
Furthermore, for the wavelength sweeping light source, one using the above-described pulse light source and also one using CW light (continuous wave: continuous wave oscillation light) are conceivable, but a light source having a wide sweeping range and a sufficiently high sweeping speed is not obtained in an actual situation.